Tested several different combinations today - footage from the GF3 and GH2, both in 1080p mode, and both AVCHD, but one camera was shooting 24 fps and one was 30. Final Cut had no problem importing and working with the different frame rates (well, it actually crashed the first time when I tried to import the 2nd card, but after that it went smoothly). Quicktime plays it no problem. I opened the inspector to see what framerate it was using and it seems to have exported the whole thing at 24fps, which was the setting from the first piece of footage. So it seems it used that info to create the project and conformed everything to it. I even tried uploading it to youtube and it worked flawlessly.
I also tried combining AVCHD and Mjpeg footage, both shot on the GF3 and at the same size and frame rate. That also worked, though I didn't try uploading it to youtube.
Since last night I've been trying to figure out how I was getting 24p from the GF3 - can't replicate it today. Possibly I was just mistaken, or maybe it was in one of the photo modes (you can set it up in stills mode and press the dedicated video button and it will capture video using the stills settings). I'll keep messing with it and if I find it I'll write it down before I forget.
Anyway, now I know I can use a 2-camera setup to record things and the footage will combine flawlessly into a single video -- as long as it's all the same size (didn't even try combining different sizes).
One thing I've discovered is that I now need to come up with a much better system for logging my footage and storing it - I literally need to write down what I'm recording on each card and create a nested system of folders on a scratch disc to contain it all, as well as a systematic naming convention for it all. It's weird - adding just 1 camera to the production line suddenly multiplies the logistical difficulty by several factors.